This invention relates to diagnostic ultrasound imaging, and, more particularly, to a system and method for automatically selecting the position of one or more focal positions of a transmitted ultrasound beam.
Ultrasound can be used to image tissues and vessels using a variety of imaging modalities. For example, B-mode scanning can be used to image tissues by portraying the tissues in a gray scale in which the brightness of each region of the image is a function of the intensity of ultrasound returns from corresponding regions of the tissues. B-mode scanning can be used to visualize the shapes of organs and vessels, and to detect the presence of masses, such as tumors, in tissues. Doppler scanning can be used to provide images showing the velocity of moving sound reflectors, such as blood flowing through an artery or vein. Using Doppler scanning to image the flow pattern of blood through a vessel allows the internal shape of the vessel to be inferred. As a result, partial obstructions in blood vessels can be detected.
A conventional diagnostic ultrasound imaging system 10 is shown in FIG. 1. The ultrasound imaging system 10 includes a scanhead 20 having a transducer face that is placed in contact with a target area containing tissues, organs or blood vessels of interest. As explained below, the scanhead 20 includes an array of transducer elements 24 each of which transforms a transmit signal into a component of an ultrasound beam and transforms an ultrasound reflection into a respective receive signal. These signals are coupled between the scanhead 20 and an imaging unit 30 through a cable 26. The imaging unit 30 is shown mounted on a cart 34. The imaging system also includes a control panel 38 for allowing a user to interface with the system 10. A display monitor 40 having a viewing screen 44 is placed on an upper surface of the imaging unit 30.
In operation, the transducer elements 24 in the scanhead 20 collectively transmit a beam 50 of ultrasound energy as shown in FIG. 2. Respective electrical signals, typically at a frequency of 1-20 MHz, are applied to all or some of the transducer elements 24. The number of transducer elements 24 to which electrical signals are applied determines the size of the transmit aperture. The size of the aperture affects the size of the imaging field and resolution, as explained below. In practice, the phases of the electrical signals applied to the transducer elements 24 are adjusted so that the beam 50 is focused in a focal position 52. The depth to the focal position 52 beneath the transducer face is controlled by the magnitude of the differences in phase of the electrical signals applied to the transducer elements 24. The focal length, which corresponds to the effective length of the focal position 52, is determined by the size and gain of the transmit aperture, i.e., the number of transducer elements 24 used to form the beam 50. The focal position 52 should ideally be positioned where features of maximum interest are located so that these features will be in the best attainable focus. The focal position 52 is shown for illustrative purposes in FIG. 2 as being considerably xe2x80x9csharperxe2x80x9d than typical in practice. The ultrasound from the individual transducer elements 24 is normally diffracted by tissues so that the effective length of the focal position 52 is actually more of an area where the beam 50 is narrowed rather than a location where the beam 50 comes to a point.
As previously mentioned, the transducer elements 24 are also used to receive ultrasound reflections and generate corresponding electrical signals. As shown in FIG. 3, the phase and gain of the received signals are also adjusted to effectively generate a receive beam 56 that is focused to a focal position 58 corresponding to the phase differences between the signals coupled from the transducer elements 24. (In the interest of clarity, beam components for only two transducer elements 24 are shown, although it will be understood that beam components would exist for all active transducer elements). The receive beam 56 can also be xe2x80x9csteered,xe2x80x9d i.e., offset from an axis that is perpendicular to the transducer face, by adjusting the phase differences between the signals coupled from the transducer elements 24. In practice, the phase differences between these signals are adjusted as a function of time delay from each ultrasound transmission so that the focal position 58 dynamically varies with depth from a relatively deep position 60 to a relatively shallow position 62 from where the ultrasound is reflected. Thus, in contrast to the constant position of focal position 52 for the transmit beam 50, the focal position 58 for the receive beam 56 varies dynamically with the depth from where the ultrasound is reflected. As explained below, the disclosed invention relates to the locations of the focal position 52 for the transmit beam 50 rather than the locations of the focal position 58 for the receive beam 56.
A typical B-mode ultrasound image 64 is displayed on the viewing screen 44 as shown in FIG. 4. The ultrasound image 64 shows a number of anatomical features, such as tissues 66 and a blood vessel 68. In the specific case shown in FIG. 4, the area of interest to the medical practitioner is the vessel 68. As a result, the focal position of the transmit beam should ideally be located at the depth of the vessel 68. The conventional ultrasound imaging system 10 (FIG. 1) has the ability to adjust the location of the transmit beam focal position. As shown in FIG. 4, the location of the focal position along the depth axis of the image 64 is indicated by a cursor 70 on the right hand side of the viewing screen 44. The location of the focal position is adjusted by suitable means, such as by manipulating a control on the control panel 38 (FIG. 1). As a result, a medical practitioner can place the focal position of the transmit beam at the area of greatest interest in the ultrasound image 64.
It is possible for objects of interest to be larger than can be effectively focused by a single focus position, or that there are multiple objects at different depths of field, which cannot be adequately focused by a single focus position. One solution to this problem is provided by the conventional ultrasound imaging system 10 generating an image using two or more transmit focal positions, as shown in FIG. 5. The viewing screen 44 shows a B-mode image 80 showing tissues 82 containing a relatively large blood vessel 84. A single focal region may be too small to optimally image the vessel 84. For this reason, a medical practitioner has the option of selecting a number of transmit focal positions, e.g., two focal positions as indicated by the cursors 86, 88 on the right hand side of the viewing screen 44, as shown in FIG. 5. The positions of the focal positions are adjusted by suitable means, such as by manipulating a control of the control panel 38.
The two transmit focal positions are used by first transmitting a beam of ultrasound focused at the first focal position. Ultrasound reflections are then obtained as explained above, and a first set of data corresponding thereto are stored by suitable means. A second beam of ultrasound focused at the second focal position is then transmitted, and ultrasound reflections are then also obtained and a second set of data corresponding thereto are stored. The image 80 is then formed using both sets of stored data, with the portion of the image in the first focal position predominantly derived from the first set of data and the portion of the image in the second focal position predominantly derived from the second set of data. A preferred way to employ multiple transmit focal regions is described in U.S. Pat. No. 6,315,723.
The operation of the system 10 has been explained with reference to the B-mode images shown in FIGS. 4 and 5. However, it will be understood that the same principles apply to other types of images, such as Doppler images.
Although the system 10 can be operated as explained with reference to FIGS. 4 and 5 to optimally position the transmit focal position(s), it nevertheless has its limitations and problems. For example, it can be fairly time consuming to place the transmit focal positions in the correct position. Additionally, it can require an extraordinary level of expertise to select the proper number of focal positions and correctly position each of the focal positions at the optimal location. For these and other reasons, the focal position(s) are often not positioned in the optimal location, and, in many instances, practitioners do not even attempt to optimally position focal positions. In fact, practitioners are sometimes not even aware that the position of the focal position can be adjusted or that multiple focal positions can be used. There is therefore a need for a system and method that can quickly and easily select the optimal number of focal positions and their optimum positions without the need for extraordinary operating expertise.
An ultrasound diagnostic imaging system and method uses an image processor that automatically sets the location of a focal position of the beam of ultrasound transmitted by an ultrasound scanhead based on an analysis of an ultrasound image displayed on an ultrasound display. The image processor may analyze the image to automatically identify an area of interest, or the area of interest may be selected manually by a user. The image processor may automatically identify the area of interest by analyzing a characteristic of the image, such as the quality of the image. The image processor may set the location of the focal position to correspond to the position of an area of interest, to maximize the quality of the ultrasound image in the area of interest, or by some other means. The image processor may also select the number of ultrasound transmissions and the locations of respective focal positions based on an analysis of the ultrasound image. The image processor may also dynamically vary the position of a focal position by varying the location of the focal position along a depth axis as a function of the locations of areas of interest along an azimuth axis of the ultrasound display.